1. Field of the Invention
The present invention relates to a power device integrated structure with low saturation voltage.
2. Discussion of the Related Art
It is known that the higher the drain-source voltage a power MOSFET must withstand, the thicker the epitaxial drain layer wherein the power MOSFET elementary cells are formed must be. This layer is lightly doped, and has a resistivity of the order of 50 Ohm/cm; since for maximum voltage ratings of 1000-2000 V a 100 .mu.m-thick layer must be provided, a significant resistance is introduced in series to the conductive channel, which contributes to increasing the power MOSFET on-state resistance Ron. The power MOSFET Ron is thus directly related to the power MOSFET breakdown voltage BV, so that the higher BV the higher Ron. This is disadvantageous since it causes significant voltage drops across the power device, and large power dissipations.
The Insulated Gate Bipolar Transistor (IGBT) has been introduced to overcome the limitations of the power MOSFET structure in the high-voltage (1000-2000 V) application field.
The fundamental idea that led to the IGBT is the introduction of conductivity modulation of the drain layer: to this purpose, the substrate of the chip is doped of with a conductivity type opposite to that of the drain layer, so as to obtain a PN junction. When the IGBT is turned on by applying a proper bias voltage to the gate electrode, carriers flow from the emitter electrode through the conductive channel, and drift through the drain layer (which for this reason is also called "drift layer"); the substrate-drift layer junction becomes forward biased, and carriers of the opposite sign are injected from the substrate into the drift layer, whose conductivity is thus enhanced.
The problem is that the IGBT structure has inherently associated a parasitic three-junction device (i.e. a thyristor). Considering, for example, an N-channel device, the parasitic thyristor is made up of an NPN transistor with emitter and base respectively formed by the source and body regions of the elementary cells, and a collector formed by the drift layer, and a PNP transistor with collector, base and emitter respectively represented by the body regions of the elementary cells, the drift layer and the substrate.
If the parasitic thyristor triggers on, a low-resistance path is established between the source regions and the substrate, the current flowing through the device theorically diverges, and the IGBT is destroyed. It is therefore extremely important to prevent the parasitic thyristor from triggering on.
To this purpose, the designers' efforts conventionally aim at preventing the parasitic NPN from entering conduction. One way to achieve such a result is by reducing as far as possible its common base current gain .alpha.n. The dopant concentration in the source regions and in the body regions of the elementary cells, which respectively form the emitter and the base of the parasitic NPN, is thus properly chosen so that the difference in said concentration does not exceed a given value. Another way to assure that the parasitic NPN does not enter conduction is to clamp its base-emitter voltage to zero: the body regions are thus short-circuited by the emitter metal layer to the source regions. Also, to reduce the base-emitter resistance Rbe of such transistor, i.e. the physical resistance along the body region of the elementary cells, the body regions are provided with a heavily doped portion, and the length of the source regions is controlled so that they are almost completely internal to the heavily doped portion of the body regions.
The equivalent electrical circuit of the structure thus obtained includes a power MOSFET and a PNP transistor (the MOSFET's drain and source being connected to the PNP's base and collector, respectively), since the NPN transistor is substantially eliminated.
In this way, however, conductivity modulation of the drift layer can only take place from the substrate. Since the average lifetime, and consequently the diffusion length, of holes in the drift layer is low (to increase the IGBT speed performances the lifetime of holes in the drift layer is deliberately lowered by the introduction of lifetime killers, and a heavily doped "buffer" layer is provided at the bottom of the drift layer to facilitate the collection of holes when the IGBT is switched off), the conductivity modulation effect is quite limited, and the output resistance is high. In these conditions, it can be shown that the collector-emitter voltage in saturation (VCEsat) is proportional to the square of the drift layer thickness, and is therefore quite high. This causes static losses in the device.
In view of the state of the art described, it is an object of the present invention to provide a power device integrated structure in which the collector-emitter saturation voltage is lower than in conventional IGBT structures, so that static losses can be reduced.